Prognostic Meteorological Discussion
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000
FXUS02 KWBC 201602
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1202 PM EDT FRI OCT 20 2017

VALID 12Z MON OCT 23 2017 - 12Z FRI OCT 27 2017

...OVERVIEW...

GUIDANCE IS SHOWING A HIGHER DEGREE OF VARIABILITY AS THE EASTERN
U.S. RIDGE IS FORECAST TO BE REPLACED BY WAVES OF ENERGETIC HIGH
AMPLITUDE TROUGHS NEXT WEEK AS WE HEAD DEEPER INTO FALL.
CYCLOGENESIS APPEARS LIKELY FROM THE EASTERN GULF COAST UP TOWARD
THE APPALACHIANS AND LOWER GREAT LAKES SOME TIME NEXT TUESDAY.
THIS SHOULD BE FOLLOWED BY ANOTHER SIGNIFICANT TROUGH DROPPING
SOUTH FROM WESTERN CANADA INTO THE PLAINS AND THE GREAT LAKES BY
THE LATTER HALF OF NEXT WEEK.


...GUIDANCE EVALUATION/PREFERENCES...

THE FIRST FEATURE OF CONCERN IS THE POTENTIAL FOR CYCLOGENESIS AT
THE START OF THE MEDIUM-RANGE PERIOD OVER THE EASTERN U.S.  MODELS
ARE INDICATING THAT A FRONTAL WAVE APPROACHING THE SOUTHEAST U.S
EARLY NEXT WEEK WILL AMPLIFY AS AN ENERGETIC UPPER TROUGH
APPROACHES FROM THE PLAINS.  THE ECMWF HAS BECOME MORE AGGRESSIVE
WITH THE RATE OF CYCLOGENESIS FOR THE LAST COUPLE OF RUNS WHEREAS
THE GFS HAS BEEN QUITE CONSISTENTLY INDICATING A LATER ONSET OF
THE CYCLOGENESIS.  DESPITE THIS DIFFERENCE...BOTH MODELS ARE
INDICATING A STRONG COLD FRONT MOVING ACROSS AND OFF THE EAST
COAST BY WEDNESDAY.

MEANWHILE...MODELS ARE INDICATING THE NEXT SIGNIFICANT UPPER
TROUGH ACROSS WESTERN CANADA SHOULD BE DIPPING INTO THE NORTHWEST
AND NORTHERN U.S. DURING LATTER HALF OF NEXT WEEK.  MODELS HAVE
TRENDED TOWARD A MORE AMPLIFIED SOLUTION WITH THE COLD AIR MASS
REACHING FARTHER SOUTH INTO THE PLAINS LATE NEXT WEEK.  THERE IS
STILL A HIGH DEGREE A UNCERTAINTY ON WHETHER A CYCLONE WILL
DEVELOP ALONG THE FRONT FROM THE CANADIAN BORDER AND HEADING
FARTHER INTO CANADA.

NOTE ALSO THAT THE EXTRATROPICAL TRANSITIONS TYPHOON LAN MAY LEAD
TO DOWNSTREAM ADJUSTMENTS IN THE CONUS THAT ARE STILL NOT YET
APPARENT.

THE UPDATED FORECAST INCORPORATES A COMPROMISE BETWEEN THE 00Z
ECMWF AND THE 06Z GFS.  THE BLEND INCREASES EMPHASIS OF THE
ENSEMBLE MEANS LATE IN THE PERIOD GIVEN THEIR RELATIVE AGREEMENT
VERSUS THE SPREAD OF OPERATIONAL RUNS.


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

MULTIPLE AREAS OF HEAVY RAINFALL ARE POSSIBLE TO THE EAST OF THE
MS RIVER AS A WAVY FRONT GRADUALLY PROGRESSES ACROSS THE EASTERN
STATES.  FLOW AHEAD OF AN INITIAL MS VALLEY UPPER TROUGH/LOW
SHOULD SUPPORT AN AXIS OF HEAVIEST ACTIVITY FROM THE EAST-CENTRAL
GULF COAST NORTHWARD EARLY IN THE WEEK WITH ATLANTIC INFLOW
EVENTUALLY SUPPORTING ENHANCED AMOUNTS OVER THE NORTHEAST MID-LATE
WEEK AS THE FRONT CONTINUES ONWARD.  DETAILS/TIMING OF INDIVIDUAL
FEATURES ALOFT ARE STILL UNRESOLVED, TEMPERING CONFIDENCE IN
SPECIFICS OF EXACTLY WHERE/WHEN HEAVIEST RAINFALL WILL OCCUR.
TOWARD FRI ANOTHER AREA OF PRECIP MAY DEVELOP WITH ANOTHER FRONT
REACHING THE EAST-CENTRAL U.S..  MEANWHILE AN AREA OF MOSTLY LIGHT
PRECIP SHOULD EXTEND FROM THE NORTHWEST INTO THE CENTRAL
ROCKIES/HIGH PLAINS AROUND WED-THU.

THE AMPLIFIED PATTERN ALOFT WILL LEAD TO SOME SIGNIFICANT
TEMPERATURE ANOMALIES.  LOCATIONS OVER CALIFORNIA AND THE
SOUTHWEST/GREAT BASIN MAY SEE ONE OR MORE DAYS WITH HIGHS OF
10-20F ABOVE NORMAL WITH A FEW DAILY RECORDS BEING CHALLENGED.
THIS HEAT ALONG WITH DRY/GUSTY WINDS ASSOCIATED WITH OFFSHORE
GRADIENTS WILL LIKELY ENHANCE WILDFIRE DANGER ONCE AGAIN.  A VERY
BRIEF EPISODE OF WARMTH MAY OCCUR OVER THE NORTHERN-CENTRAL PLAINS
AROUND MIDWEEK.  THE EASTERN U.S. WILL ALSO BE QUITE WARM AT THE
START OF THE WEEK BEFORE FRONTAL PASSAGE, BUT THEN TREND BELOW
NORMAL WITH A DAY OR TWO OF MINUS 5-15F ANOMALIES BY WED-THU.

KONG/RAUSCH

$$





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